ALTER TABLE ReturnProductDetail DROP CONSTRAINT FK_ReturnProductDetail_Product  
ALTER TABLE ReturnProductDetail DROP COLUMN ProductId

IF NOT EXISTS(SELECT [Name] FROM syscolumns WHERE Name = 'BillDetailId' AND ID = OBJECT_ID('dbo.ReturnProductDetail'))
   ALTER TABLE ReturnProductDetail ADD BillDetailId INT NOT NULL
   
ALTER TABLE dbo.[ReturnProductDetail]
	ADD CONSTRAINT rfk_BReturnProductDetail_BillDetail_FKID FOREIGN KEY
		( [BillDetailId] ) 
	REFERENCES dbo.BillDetail
		( [Id] )